1. Field of the Invention
The present invention relates, in general, to apparatus for measuring electrical energy and, more particularly, to apparatus and methods for monitoring power drawn by an electrical load.
2. Description of the Art
Various control devices have been developed to measure electrical power drawn by an electrical load, such as a motor driven spindle or axis drive in a machine tool. Particularly, control devices have been devised for measuring the spindle or axis drive power of three-phase AC motors.
Such power sensors measure or sense the motor""s power rather than current due to the linear relationship between the change in motor load and the change in motor power which is generally proportional. So by measuring changes in the power consumption of a machine""s motor, a power measurement device or sensor can provide an output signal that is extremely sensitive to machine conditions. The output signal can be communicated to a controller or intelligent tool monitor which can process the analog output signal of the power sensor to detect tool wear, a missing tool, a misplaced part, loss of load or overload.
For example, Montronix, Inc. of Ann Arbor, Mich., sells a PS100 Power Sensor for measuring spindle or axis drive power of an electric motor. Three Hall effect sensors are used to measure the current drawn by each phase of the electric motor. The analog output signals from the Hall effect sensors are combined with a voltage measurement from each supply phase to generate a DC voltage signal (the xe2x80x9ctotal power signalxe2x80x9d) that is proportional to the total power being consumed by the motor being driven. The power measurement range of the sensor is determined by plug-in range adjustment modules.
However, a problem is encountered when measuring the power drawn by the motor for light loads typically associated with small tools, such as small diameter drill bits on the order of xe2x85x9 inch diameter or smaller, for example. The low power levels drawn by the tool during the cutting or tool operation phase are typically much lower than the power drawn by the machine drive motor at idle. In such situations, the power sensor is incapable of distinguishing between the idle power drawn by the machine tool drive motor or the power drawn by the machine tool motor during a machining operation.
In an attempt to address this problem in obtaining accurate power measurements for small tools, Montronix, Inc. developed and sells an analog gain module, sold under the product name xe2x80x9cPS100 Gain Modulexe2x80x9d which is plugged into a multi-pin connector provided on the PS100 Power Sensor described above. This analog gain module includes a selectable voltage offset of 1, 2, 3 or 4 volts and switch selectable, fixed gains of 2 or 4. The use of the offset was an attempt to compensate for the idle power drawn by the machine motor.
However, while this analog gain module, in certain situations, is able to successfully monitor the power drawn by small loads, such as those associated with small tools, the offset has to be established for each different tool. The gain also has to be selected for each new tool. However, if the offset and/or the gain are set improperly, output power readings could be off of the measurement scale, thereby providing useless power measurement.
It is also known that the idle and working power drawn by a motor driven tool will vary as the tool wears, ambient temperature changes, etc. This makes the preselection of the offset and gain difficult as the idle and cutting load power may vary throughout any given time period. Further, the installation of a new tool, such a new drill bit, in a given machine has different RPM and loads during cutting, again requiring a re-selection of the offset and gain values.
Thus, it would be desirable to provide a power sensor or monitor which is capable of accurately measuring power drawn by a drive motor operating small tools. It would also be desirable to provide a power sensor capable of accurately measuring power drawn by a motor drive operating small tools which can be easily used with existing tool power monitoring systems.
A method according to the present invention is disclosed for measuring the power load of a cutting tool on a drive motor operating the cutting tool in a cutting or operating cycle having idle and machining or cutting states, the method comprising the steps of:
measuring the electrical power load on the motor continuously during one complete cycle; and
offsetting the measured electrical power values during the machining state with the measured electrical power values during the idle state to generate an output signal proportional to the power consumed only by the cutting process.
In another aspect of the invention, the present method offsets the measured power value by establishing an average power value during the idle state of the cutting cycle.
In yet another aspect, the method includes the steps of measuring the electrical power values in a setting period during the idle state of the cutting cycle, accumulating all of the power values during the time period, and determining the average electrical power value over the reset time period.
Typically, the offset value is measured and averaged each time a different machining operation is begun. Preferably, this occurs after the machine tool motor has reached cutting speed and stabilized, but before the particular tool has begun cutting.
In yet another aspect, a filter is employed to smooth out noise in the raw power signal. Another filter is employed to smooth out noise in the output signal.
In another aspect, the method includes the step of setting a variable selectable gain to a multiplier. The gain is selectable between 2 and 800X, for example. The filters and selectable gain, individually and in combination, vastly improve the signal-to-noise ratio of the output signal which makes the difference between a useful and a useless power measurement system.
In another aspect of the inventive method, the measured power is input to a time-domain averaging means which accumulates the power input signals and averages the signals to compute a new average offset value. This time-domain averaging feature uniquely enables a new offset value to be calculated at any time, such as at the start of each machining cycle or whenever a different tool is used within a machining cycle.
In another aspect of the invention, an apparatus is disclosed for measuring the electrical power load drawn by a machine tool drive motor in a cutting operation or cycle including idle and machining or cutting states. The apparatus includes means for receiving the measured power load on the motor continuously during one cutting cycle, means for calculating an offset value corresponding to the measured power value during the idle state of the cutting cycle, and means for outputting power values proportional to the electrical power consumed during the cutting process wherein the output power values are offset from or have the offset value subtracted from the aggregate power.
According to another aspect of the present invention, the means for calculating the offset value establishes an average power value during the idle state of the cutting cycle. Specifically, the calculating means includes means for measuring the electrical power values in an averaging period during the idle state, means for accumulating all of the power values during the averaging period, and means for determining the average electrical power value over the averaging period.
According to another aspect of the present invention, the apparatus includes a filter which smooths out noise in the raw power signal. Another filter is employed to smooth out noise in the output signal.
According to another aspect of the invention, the apparatus includes a variable selectable gain which is selectable between 2 and 800X, for example.
The present invention overcomes several deficiencies found in prior art power measurement devices used to measure the electrical power or load on a motor driven machine tool or other electrical device. The present power measurement or sensor apparatus precisely measures the very small increase in motor power consumption resulting from the cutting action of small tools. These small increases in power consumption have heretofore been masked by the normal idle power drawn by the motor. The present power gain apparatus uniquely calculates an offset corresponding to the average idle power at each use of a different tool or, alternately, at the start of each machining cycle, thereby accommodating tool wear, ambient temperature changes, etc. The offset is subtracted from the aggregate power measurement to yield an output signal proportional to the incremental power drawn by the cutting tool. The apparatus of the present invention is easily connectable to existing power sensors thereby enabling its use in a large number of existing applications as well as in new applications.
The use of the filter(s) and the selectable gain, individually or in combination, vastly improves the signal-to-noise ratio of the output signal which can make the difference between a useful power measurement system and a useless one.
The power measurement apparatus of the present invention may be uniquely mounted at the location of the power sensor itself. This enables the apparatus to process signals before transmission to a remote location as in prior art power measurement apparatus thereby improving the sensitivity, or signal-to-noise ratio, of the power measurement.